how to use the ComputedField?

in the Orchard.ContentManagement.Utilities path,there is a class named ComputedField.cs.
now I have a problem as follow:
a FixedAsset class,it have some property,they are AssetName,AssetFirstValue,BuyDate,for example AssetName is IBM Mobile,BuyDate is 2013-1-1, and I want to copute the Month of the IBM Mobile have used utill now(that is 5 months), as the time passed, the Months
of the Asset been used may change ,so how to model the FixedAsset if I want to use ComputedField or have any good idea for this problem? thans!